1. Field of the Invention
The present invention relates to the field of thin layer formation and more particularly to a method and apparatus for forming silicon nitride layers.
2. Discussion of Related Art
Modern integrated circuits are made up of literally millions and millions of transistors integrated together into functional circuits. In order to further increase the computational power or storage capability of integrated circuits, transistor feature size such as gate length and gate oxide thickness must be further scaled down. Unfortunately, as transistor gate lengths are continually scaled, the transistor's electrical characteristics and performance can greatly change due to thermal redistribution of dopants in the device. As such, as devices are further scaled, the thermal budget, i.e., the individual process or cumulative heat input from deposition and process temperatures, used to manufacture the integrated circuit must also be reduced to insure consistent and reliable electrical performance of the device. Additionally, in order to further scale the semiconductor device, the thin layers used to make the devices must be able to be formed with high compositional and thickness uniformity.
One material used in the formation of transistors is silicon nitride. Silicon nitride thin layers are conventionally deposited by thermal chemical vapor deposition (CVD) in semiconductor fabrication processes. For example, silicon nitride layers are used as spacer layers, etch stops, as well as capacitor and interlayer dielectrics. However, present techniques of forming high quality silicon nitride layers in a single wafer reactor utilizing thermal chemical vapor deposition require high deposition temperatures of greater than 750° C. and/or have reduced deposition rates at reduced temperatures, and can result in no appreciable deposition of silicon nitride for transistor fabrication.
Additionally, when silicon nitride layers are deposited at reduced temperatures or at high deposition rates with current processes and precursors, the quality of the layer is generally less than desirable. For example, current silicon nitride precursors including silane, dichlorosilane, disilane, bis-tertbutylaminosilane (BTBAS), and hexachlorodisilane have produced layers with less than desired layer quality, such as low density and high hydrogen content. Disilane and hexachlorodisilane have weak Si—Si bond which allows for acceptable deposition rates, but when used with a nitrogen source such as ammonia either lead to poor film quality (low density and high hydrogen content for both, and poor step coverage and microloading for disilane) or almost unmanageable particle generation (for hexachlorodisilane).
Thus, what is needed is a method of forming a high quality silicon nitride layer by thermal chemical vapor deposition (CVD) at reduced deposition temperatures at a manufacturable deposition rates.